Often, electrical loads are operated using a main or usual source of power and a back-up power supply, such as a battery or an uninterruptable power supply (UPS). As is well known, back-up power supplies typically only provide a limited amount of power for a limited time. Accordingly, the loads are often driven at a lower load level when the back-up supply is operative, so that the loads can operate for a longer period of time. Further, it is often desirable to operate the loads in a manner that indicates when the back-up supply is providing power, so that corrective action can possibly be undertaken to restore main power, such as from an electric power utility.
In the case of a lighting load, such as luminaires utilizing LEDs, the load can be operated for a relatively long period of time due to the low power requirement of such a load as compared to other loads. However, even in such a case, it is desirable to operate the load with reduced power output to preserve battery life and to indicate that power is being drained from a limited power source. Accordingly, a known approach to operating a dimmable LED load, for example, in a public location, has been to provide a circuit remote from the LED load and coupled to main and auxiliary (i.e., back-up) power inputs. The circuit provides appropriately conditioned power to the luminaires, and further develops a dimming signal on a dedicated dimming line that commands the luminaires to operate at a fully energized state when main power is being provided and which commands the luminaires at a dimmed state (or operates some of the luminaires in the dimmed state while remaining luminaires are turned off) to preserve power when auxiliary power is being provided. Typically, a relay is activated when the power source is switched from the utility to the back-up power source, and the relay shorts the dimming signal so that the lighting fixture senses 0 volts. Providing 0 volts to the lighting fixture causes the lighting fixture to drive the illumination device at a predetermined minimum output level.